void main() {
    //渐变方块点
    //gl_PointCoord类似于UV（y轴朝下）
    //gl_FragColor = vec4(gl_PointCoord, 0.0, 1.0);

    //渐变圆点
    // float strength = distance(gl_PointCoord, vec2(0.5));
    // gl_FragColor = vec4(strength, strength, strength, 1);

    //白色圆点
    // float strength = 1.0 - distance(gl_PointCoord, vec2(0.5));
    // strength = step(0.5, strength);
    // gl_FragColor = vec4(strength, strength, strength, 1);

    //朦胧圆点
    float strength = distance(gl_PointCoord, vec2(0.5));
    strength = strength * 2.0;
    strength = 1.0 - strength;
    gl_FragColor = vec4(strength, strength, strength, 1);
}